Well known in the art are contacting calibration devices with roller arms as calibration means, which are adapted via different adjustment devices to the diameter of the blown film tube. In other design principals, as for example described in DE 26 38 744 A1, the calibration means, contacting the film, consist of curved rods, on which a multitude of small rings is arranged, which are rotated by contact with the film. This design type is extremely prone to contamination and leaves markings or even surface damages on the blown film depending on the degree of contamination on the separation positions of the Teflon rings.
In newer improved design types, the above named embodiment of the calibration means is substituted by straight castors with ball-bearing. For example, from DE 20 2005 006 532 U1 it is known, to provide several castors with more or less horizontal axis and with essentially tangential arrangement to the round film tube, wherein the castors are arranged circumferentially distributed in several planes arranged above each other. The castors form in a top view a polygonal cross-section, which forms a feed through opening for the film tube. The castors are radially adjustable, so that the diameter of the feed through opening can be adapted to the diameter of the film tube. The castors are in contact with the film tube and are rotatably arranged for preventing markings on the film tube around the horizontal axes. For the castor surface, materials are used, which prevent an adhesion of the film tube on the castors, like for example Teflon, nylon, silicone or similar materials. Insofar a structured castor surface is required, also foam material, felt or similar materials can be used.
The film tube is extruded from a thermoplastic material and evaporates waxy components (monomers), which can settle on the castors of the calibration device. Thus, an increased resistance during the rotation of the castors is produced. Furthermore, also the encasings get dirty after a while and leave markings, insofar as no regular cleaning or exchange of the castor casings is carried out.
Furthermore, contact-free working calibration devices exist, in which instead of castors fixed, non-rotatable guide bodies are provided as calibration means, from which an air curtain is expelled from a micro-porous surface in direction of the film tube, to prevent a film contact. Such embodiments are described in detail for example in EP 1 488 910 B1 and WO 2005/084919A1.
These constructions are extremely prone to contamination, as the porous surface clogs up easily because of the oily monomers, evaporating during the extrusion process from the melt film in connection with the dust from the surrounding air. Furthermore, the complete prevention of contact is not always ensured, as the velocity vector of the diffuse exiting air curtain, directed at a right angle against the film tube, is small and is not always sufficient as a counter force, when the film tube is agitated in plants with high performance by the velocity of the cooling air, expelled in the tube formation zone to high frequency movements.
For preventing these disadvantages EP 0 143 154 A1 proposes a calibration device, which has an annular support element, which encloses the film tube externally. The support element is provided with ring channels open towards the inside and which have ducts for feeding air. The ring channels are limited by flanks, wherein such an air amount is blown into the ring channels, that the air flowing away via the lateral flanks forms an air cushion supporting the drawn-off film tube. Thus it is ensured, that the film tube does not contact the support element.
DE 36 37 941A1 shows a device for post-cooling and calibrating guiding of a film tube above the glass transition temperature boundary of the film tube. This comprises a hose ring, which encloses and does not touch the film tube and which on its inner side, directed toward the film tube has blow bores, which are directed onto the film tube. The hose ring can, in this case, when seen in circumferential direction, consist of individual arcuate portions and can be manufactured from a material, which enables a deformation and thus an adaptation of the hose ring to different diameters of the film tube.
A calibration device of the above named type is disclosed in EP 0 273 739 A1. The calibration device comprises a calibration ring, which encloses completely the film tube, or several calibration segments, which extend along a portion of the circumference of the film tube. These have respectively two slot-like openings, directed inward, wherein one of the slot-like openings is aligned in the drawing-off direction of the film tube and one of the slot-like openings is aligned against the drawing-off direction of the film tube. The two slot-like openings are, furthermore, directed away from each other, so that a zone with low pressure or a zone with a partial vacuum is produced between the two openings. This zone with low pressure serves to attract the film tube to the calibration segment, wherein the air flows exiting the slot-like openings, form respectively an air cushion, which prevents, that the film tube contacts the calibration segment. Furthermore, the surface of the calibration segment in the area of the zone with low pressure is set back relative to the other surfaces of the calibration segment, facing the film tube, so that also because of this it is ensured, that the film tube does not contact the calibration segment.
Disadvantageous in the known embodiments is, that the air is fed in a direction transversally to the surface of the film tube, whereby a direct force is acting on the film tube, which can lead to a deformation or also to an instability of the film tube. Or zones with low pressure are present as according to EP 0 273 739 A1, which, according to the experience made, can lead to a pulsating movement of the film tube, what again effects the stability of the guidance of the film tube.
The object of the present invention is to propose a calibration device, which ensures a stable guidance of the extruded film tube.